Over time the surgical technique for total hip replacements has evolved. Incision length has been reduced over time as surgeons become more comfortable operating with limited visibility. The location of the incision has also been changing as surgeons have developed and implemented different approaches to the joint. These two factors have increased the challenges of implanting acetabular implants in the correct orientation as the acetabular tools impinge on bone or soft tissue around the perimeter of the incision.
Acetabular implants normally consist of a shell or cup and a modular insert that fits within the shell and acts as a bearing surface for the femoral head. While modular shells and inserts are preferred for a number of reasons, there is an application where shells and inserts are combined preoperatively in a monoblock construction. Such shells and inserts are shown in U.S. Pat. No. 6,475,243, the disclosure of which is incorporated herein by reference.
Acetabular instruments normally consist of a series of reamers, a reamer handle, a shell positioner/impactor and an insert positioner/impactor. In addition, alignment guides are often attached to the reamer handle and shell positioner/impactor in order to facilitate alignment. A typical reamer is shown in U.S. Pat. Nos. 4,023,572 and 5,658,290.
Shells are implanted into an acetabulum after the acetabulum has been prepared to receive the shell through the use of a series of reamers increasing in size. The shells are aligned in the acetabulum according to two angles: abduction and anteversion. The combination of these two angles creates the axis that the shell should be aligned and impacted on.
Traditionally acetabular reamer handles and shell and insert positioner/impactors had straight shafts. In some surgeries the size or location of the incision results in the shaft of these instruments impinging on the side of the incision before the preferred abduction/anteversion axis is achieved. In these cases the surgeon has to force the soft tissue or bone out of the way, increase the length of the incision, or accept the abduction/anteversion angle that can be achieved. None of these options are preferred.
One method for avoiding impingement between these acetabular instruments and the incision is to create inline or offset curved acetabular instruments. “Inline” refers to an acetabular instrument that has a curved section between the two ends of the instrument that lie on the same axis. Typically, the first end includes a hex drive for connection to a rotary power service (drill) and a second end which has a holder for an acetabular instrument such as a reamer or the acetabular shell itself. “Offset” refers to an acetabular instrument that has a curved section between the two ends of the instrument that lie on different axis. The curved section should begin as quickly as possible after the attachment to the reamer, shell or insert in order to minimize the impingement.
An inline curved or offset acetabular instrument is preferred for a number of reasons. Typically, the surgeon is used to operating with inline straight instruments. By maintaining the inline aspect of the design, the ergonomics of the instrument remain the same and the surgeon learning curved is reduced. In addition an inline instrument allows for all forces to be projected in line or parallel to the correct axis. Finally, an inline instrument allows for alignment guides to be indexed around the axis of the instrument without changing its position in relation to the implant. In-line or offset reamers/impactors are shown in U.S. patent publications 2003/0050645, 2003/0229356, 2004/0153063, 2004/0087958, 2005/0038443 and 2005/0216022.
The introduction of a curve into acetabular instrument introduces various challenges to designing a reamer handle. Reamer handles are used to transmit torque and axial load from a power source, such as a rotary power source to the reamer. This is accomplished through the use of a straight shaft with a fitting that mates with the power source on one end and a locking mechanism that connects to the reamer on the other end. The torque is transmitted from the power source to the fitting to the shaft to the locking mechanism and finally to the reamer. The axial force is transmitted from the surgeon to the power source to the shaft to the locking mechanism to the reamer.
The same holds true for the shell positioner/impactors. Shells are typically connected to positioner/impactors by threading the two together with torque. This is accomplished through the use of a straight shaft with a handle that the surgeon grips on one end and a threaded fitting that connects to the shell on the other end. The torque is transmitted from the surgeon to the handle to the shaft to the threaded fitting to the shell. The axial force is transmitted from the surgeon to the mallet to the handle to the shaft to the threaded fitting to the shell.
With an inline or offset curved acetabular instrument the torque and axial load needs to be transmitted around a curve. In addition, the curved body that transmits the axial load cannot also transmit the torque. This is because the curved body would impinge on the incision if one tried to rotate it through a full rotation. To solve this problem, a handle having a hollow curved body with a drive train housed internally is provided. The drive train needs to be able to transmit torque through the curved body. This is accomplished through the use of U-joints and/or flexible shafts.
In the case of an offset curved instrument, two u-joints allow the transmission of torque between the two ends of the instrument. In the case of an inline curved instrument a series of U-joints (5-6) or a combination of U-joints (2) and a flexible shaft allows the transmission of torque between the two ends of the instrument. There are many other combinations and permutations of U-joints and flexible shafts that would meet the requirements.
The curved body is preferably machined from a solid block referred to as a monoblock, to produce the curved body rather than using a bent tube. The use of a machined body allows for increased precision of the instrument and drive train. The internal drive-train can easily be removed without the use of tools once a moveable lid is opened. This is because the multiple U-joint drive shaft is flexible and is held at position at the ends within fixed U-joints mounted to the curved body only by mating hex couplings. These couplings may be easily slid apart to disassemble the drive-train from the handle. The monoblock also serves to mount modular navigation and alignment guides such as a typical mechanical guide or a well-known optical navigation tractor. Obviously if no guide is necessary, the mounting position on the monoblock curved body can be left empty.
The modular alignment system can be attached to the body of the instrument preferably adjacent the handle. This alignment system is indexible. This allows the mechanical or optical tracker system to be positioned properly with respect to the typical acetabular cup or shell which has a plurality of screw holes therethrough. The indexible alignment system also allows for left and right positions for the impactor/reamer and allows proper positioning of the curve and minimization of impingement with adjacent soft tissue.
In one possible embodiment a modular cup or reamer holder includes an actuatable release mechanism which can be connected the curved impactor/reamer by a push button mechanism described below. The cup or reamer holder can then be attached and/or disconnected such as by threading a threaded projection on the leading end of the modular holder into a threaded bore in the cup. This can be accomplished by turning a knob at the end of the handle which turns the drive-train which in turn rotates the threaded tip. Disconnection is accomplished by turning the knob at the end of the handle in the opposite direction. The design can be reversed with the actuatable release mechanism being located on the end of the handle. This simplifies the design of the modular cup or reamer holder and reduces cost since only one release mechanism is necessary when multiple holders are used.
If a modular holder is utilized, the cup initially could be threaded onto the holder or tip while disconnected from the curved positioner or impactor. The holder cup combination can then be inserted into the hip joint, perhaps sideways as for a smaller incision, and then reconnected to the curved positioner. The modular holder can be removed from the cup and incision if the curved impactor fails during impaction. The user can disconnect the modular cup holder and utilize a new impactor by reconnecting the impactor end to the modular cup holder or tip.
This modularity also allows a user to have multiple holders designed both for preassembled metal shells and polyethylene or ceramic bearings. Thus one holder can be designed to engage the polyethylene bearing of a preassembled acetabular cup and a second modular holder designed to engage a cup design having an outer shell and a ceramic bearing. Obviously, one-piece prosthetic cups made of polyethylene or ceramic could also be gripped. These modular holders can be supplied as part of a kit which has one positioner/impactor and several holders designed to couple to different implants rather than having multiple positioner/impactors.
The preferred embodiment of the shell positioner/impactor of the present invention uses a series of U-joints. Two U-joints are fixed in the corners (one at each end) of the curved body and a chain composing of preferably four U-joints is assembled between the two end U-joints. The preferred U-joint chain is assembled by connecting two male hex fittings to female counterparts of the two corner U-joints. The U-joint chain is preferred because it transfers torque rigidly and is easy to clean.
An alternate embodiment uses a wound wire flexible shaft between the two corner U-joints. The wire flexible shaft is assembled to the two corner U-joints with the hex fittings. While the wire flexible shaft has a lower cost, it is less preferred due to spring-like torque transfer (i.e. the shaft twists before torque is transmitted) and typical flexible shafts have cleanability problems.
While the drive train can be mounted externally on the curved body or internally in the curved body, it is preferred to mount the drive train internally for a number of reasons. The drive train rotates at high speeds in the reamer handle and there would be a risk to the surgeon if it were mounted externally. Additionally, it is possible for the drive train to catch on soft tissue if it is mounted externally. Also, external mounting increases the amount of instrument components in the surgeon's field of view, and consequently decreases the surgeon's view of the incision.
The drive train can be mounted permanently or it can be removable. The preferred embodiment is to have a removable drive train mounted internally. This allows for the drive train to be replaced as it wears out or becomes obsolete without replacing the entire instrument. In addition it allows the drive train to be removed for cleaning.
The drive train is captured inside the curved body through the use of the curved lid. The curved lid can be assembled and disassembled from the curved body so that the drive train can be removed for cleaning or replacement. The monoblock curved body and lid construction is preferred over a curved tube design. A solid curved tube with an internal drive train without a lid for access cannot easily be cleaned while a tube with a curved lid can be easily disassembled from the curved body so that the internal components like the U-joints and drive train can be easily cleaned. The curved lid adds strength to the curved body. The curved body and curved lid assembly can handle more axial load than an open curved body alone.
Modular holder/tips have been developed that mate with shell implants, polyethylene inserts and ceramic inserts. In the preferred embodiment, the modular tips all connect to the shell positioner/impactor in the same fashion. A spring-loaded button in the modular tip or holder deflects an internal ring to an aligned position and released after assembly to and then engages a groove on the shell positioner/impactor providing an axial capture. There are many different methods for achieving this type of connection between the modular holder and the shell positioner/impactor. A socket feature, such as in the modular tip engages a mating feature on the shell positioner/impactor providing a radial anti-rotation constraint. This allows torque to be transmitted between the curved body of the shell positioner/impactor and the modular tip body. In the preferred embody a hexagon or octagon is formed within the modular tip end. Obvious any shape which prevents radial and rotational movement can be used. Additionally in a preferred embodiment, an internal hex fitting in the modular tip engages a mating socket in the shell positioner/impactor. This allows for torque to be transmitted between the drive-train of the shell positioner/impactor to the hex fitting on the modular tip holder. Obviously the inverse of this design could also be used with the socket on the modular tip or holder.
In the preferred embodiment applying contrasting torque forces through the knob and handle of the shell positioner/impactor actuates and deactuates each modular holder/tip. A clockwise rotation of the knob results in a lock and counterclockwise rotation of the knob is used to unlock. The surgeon transmits torque internally through the instrument by turning the knob which then transmits torque to the drive-train to the hex fitting to the locking mechanism while external torque is transmitted from the surgeon to the handle to the curved body to the tip socket. These two contrasting forces allow for the actuation of the locking mechanisms from a distance. To transmit axial force the surgeon impacts the knob with a mallet which transfers force to the handle and the curved body then to the tip and finally to the implant.
In some incisions it would be preferred to introduce the shell and bearing insert without an instrument attached in order to reduce the overall cross-section of the assembly. This is challenging with a straight impactor for instance, because it is difficult to thread the straight impactor and shell together within the incision. Furthermore a straight impactor impinges on tissue making it even more difficult to align. The modular holder tip facilitates the coupling of the impactor/positioner with a cup by allowing the user to lock the holder/tip onto the implant prior to insertion, disconnect it from a shell positioner/impactor, insert the implant holder/tip assembly into the incision and reconnecting in vivo. This is because of the larger bore in the holder and the use of a quick connect feature. This method is particularly useful with the shell that can have a roughened surface that can catch on soft tissue upon insertion.
When the cup holder is designed to mate with shells with threaded drive holes a threaded stud is captured within the body of the cup holder/tip. The threaded stud can be rotated clockwise or counter-clockwise to thread onto or off of shell implants. A cup holder/tip could be designed to mate with other shell designs. Specifically, the cup tip could be modified to have an expanding collect that could engage a smooth bore feature in a shell. These cup tips would be similar to those discussed below for holding shell inserts.
Some shells have a cluster of screw holes that need to be positioned in a specific location in reference to the curved shell positioner/impactor. There are several methods for orienting the screw holes with a curved shell positioner/impactor. One method is to thread the shell on to the cup holder/tip and positioner/impactor assembly, disassemble the shell/cup holder combination from the shell positioner/impactor preferably by pressing a button, reorienting the shell with the screw holes in the correct orientation, and reconnecting the shell/cup holder to the shell positioner/impactor. Another method for orienting the screw holes requires the surgeon to thread the shell partially on to the cup tip. By holding the shell in the correct orientation in reference to the shell positioner/impactor, the surgeon can then lock the shell and cup holder together by rotating a knob.
Polyethylene insert holders are designed to mate with the bearing surface of a polyethylene bearing insert. In one typical design, a silicone ring is captured between four components: a hex fitting, a spherical head, a compression ring and a body. When the internal hex fitting is rotated clockwise the silicone ring is squeezed between the spherical head and the compression ring causing it to protrude and create a friction lock with the polyethylene insert. The friction lock allows the user to turn, push and pull the insert which facilitates the assembly of the polyethylene insert into the shell.
Ceramic insert holders are designed to mate with a titanium outer sleeve in which a ceramic bearing insert is typically mounted. A holder is provided in which a collet is captured between two components: a hex fitting and a body. When the internal hex fitting is rotated clockwise the collet is expanded and creates a friction lock with the titanium sleeve of the ceramic insert. The friction lock allows the user to turn, push and pull the ceramic insert that facilitates the assembly of the ceramic insert into the shell.
Modular alignment guides have been developed that mate with the reamer handle and shell positioner/impactor. The modular alignment guides have a mechanism that allows the user to easily connect/disconnect the alignment guides from the acetabular instrument and to index the alignment guide around the axis of the instrument into the preferred position. It is preferred to be able to switch between a mechanical and navigation-based alignment guide. A navigation guide typically has a tracker with light emitting diodes that can be tracked via an optical system in the operating room. This allows the user to determine which method of alignment will be used and assemble the necessary guide. This in turn decreases the number of instruments in surgery.
In the preferred alignment guide embodiment, the mechanism is actuated by pressing a button that forces the translation of a locking pin perpendicular to the translation of the button. With the button pressed, the user can assemble the alignment guide to the guide fixture over the machined flats of the guide fixture. After passing the flats, the alignment guide can be rotated around the guide fixture to the preferred location. Once the preferred location is reached, the button is released and a lock pin engages the mating feature such as a bore in the guide fixture. The lock pin is spring-loaded into that position. In order to remove the alignment guide the operations are reversed.
Currently alignment guides designed for curved acetabular instruments are positioned in the same plane as the curve of the curved body. It is preferred to be able to position alignment guide independent of the curved body. The curved body should first be positioned to minimize impingement and then the alignment guides can be positioned to optimize their functionality. For a mechanical alignment guide this would involve positioning it so that it is perpendicular to the floor or patient. For a navigation-based alignment guide the user would position it to maximize visibility to the navigation cameras and to optimize the weight distribution of the navigation tracker.
While in the preferred system a separate reamer handle and a positioner/impactor is provided, it may be desirable to combine the two instruments into one instrument. This might be preferred because it would reduce the number of instruments in surgery and reduce the cost and weight of instruments. This may be accomplished through the use of modular attachments on either end of a curved body. A modular reamer locking mechanism is provided that attaches to the curved body in similar fashion to the modular holders. The main difference is that the modular reamer holder would not have a contrasting torque force supplied by the curved body. The modular reamer holder rotates freely around the curved body when torque is applied internally through the drive train. A modular impact handle can also be provided that connects with the same fitting that the power reamer attaches. The locking mechanism could be similar to those used in the modular holders. This modular impact handle would apply torque internally from a knob to the drive fitting then to the drive train of the curved instrument and would apply torque externally from the handle itself to the curved body. Axial forces would be translated from the knob to the handle to the curved body.